How to back up iOS 9.1 in order to be able to downgrade to it after updating to latest iOS?

Just got a new iPAD air 2, iOS 9.1. It says I should update it to iOS 9.3.1. I want o make a complete backup of the iPAD before update so that later I will be able to revert back all changes, including the iOS update and get the iPAD to the factory settings.

I hear jailbreak for 9.3.1 is not available yet, and on the other hand, apple stopped signing iOS 9.1 and it is impossible to install or downgrade to iOS 9.1. I want to know how I can back up the current state with iOS 9.1, and later after I updated the iOS, when ever I wanted I could restore to the iOS 9.1?

thanks